Effects of urbanization on taxonomic diversity, body size, body mass and body condition of non-volant small mammals in the Coastal Savannah Ecological zone of Ghana

doi:10.21203/rs.3.rs-4870178/v1

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Effects of urbanization on taxonomic diversity, body size, body mass and body condition of non-volant small mammals in the Coastal Savannah Ecological zone of Ghana

https://doi.org/10.21203/rs.3.rs-4870178/v1

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In the context of rapidly increasing global urbanization, studying urban landscape effects on wildlife is crucial for informing biodiversity conservation in cities. Small mammals are inextricably linked to their environment and respond swiftly to habitat alterations. Yet, the response of small mammals to urbanization in the Afro-tropics is rarely investigated. Here, we assessed the influence of urbanization on taxonomic diversity, body size, body mass and body condition of small mammals in the Coastal Savannah Ecological zone (CSEZ) of Ghana. We live-trapped small mammals in farm-bushes (UFB) and a protected area (UPA) located within the city of Accra, and compared the data with conspecifics from farm-bushes (RFB) and a protected area (RPA) located within rural areas of CSEZ. The data was analyzed using Fisher’s exact test, ANOVA, ANCOVA and F-ratio test. We found that urbanization influenced small mammal composition, but not species richness and diversity. Three species, Taterilus gracilis, Graphiurus nagtglasii and Arvicanthis niloticus occurred in the urban area only. Mastomys natlensis, Mus musculoides and Tatera kempi from the urban sites were significantly smaller and in poorer condition compared with conspecifics from the corresponding rural sites. M. musculoides from the urban area weighed significantly lesser than their rural conspecifics. Our data demonstrate the varied effects of urbanization on small mammals, and highlight the importance of heterogeneous urban green areas in maintaining small mammal biodiversity in cities. Urban developers and land use planners should prioritize integrating green areas into urban planning designs in order to promote and conserve wildlife in cities.

Bergmann’s rule

credit card hypothesis

land use change

morphometrics

rodents

scaled-mass index

urban heat island

Wildlife

There has been a surge in land use intensification since the 20th century due to rapid human population growth (Sanz-Perez et al. 2022). The global human population is currently about 8 billion, and it is projected to go beyond 9 billion by 2040 and 10 billion by 2060 (United Nations 2022). Paralleling the growth in global human population is accelerated urbanization. Currently, 55% of the global population lives in urban areas and this is projected to increase to about 70% by 2050 (United Nations 2019). As the global human population continues to increase, the rate of urbanization will escalate (Fingland et al. 2021; United Nations 2022), posing serious threat to global biodiversity and associated ecosystem services (McCleery et al. 2014; Millard et al. 2021; Sanz-Perez 2022). The effects of urbanization on biodiversity will be particularly severe in developing countries where native species are already facing a myriad of anthropogenic threats such as habitat loss and fragmentation, invasive species, pollution and climate change (Piano et al. 2020).

Although urbanization have detrimental effects on wildlife, the response strategies of wild animals differ among individuals and taxonomic groups because of differences in species’ ecological and life-history traits (Putman et al. 2019; Fingland et al. 2021). The negative effects of urbanization may be immediate for habitat specialist species (urban avoiders), and may elicit migration or local extinctions (Sol et al. 2014). However, habitat generalist species may adapt (urban adapters) or even proliferate (urban exploiters) in urban landscape during different temporal lags (Robleño et al. 2017; Fingland et al. 2021). Under natural conditions, the preferred habitat of wild animals should enhance their fitness, but habitat selection in urban landscape may be misguided because cues for high quality habitat could be camouflaged by anthropogenic activities (Sanz-Perez 2022). To achieve high level performance, wild animals respond to environmental stressors in cities by adjusting their behavior, physiology and/or morphology (Lyons et al. 2017; Putman and Tippie 2020; Ritzel and Gallo 2020; Tranquillo et al. 2023; Naidoo et al. 2024).

Morphological divergence among populations in response to urbanization is widespread, but the trait change direction varies among species and geographic locations (Iglesias-Carrasco et al. 2017; Santini et al. 2019; Ritzel and Gallo 2020; Naidoo et al. 2024). For example, birds in more urbanized habitats have shorter beak length, tarsi and tails than conspecifics from rural habitats (Giraudeau et al. 2014; Miller et al. 2018; Avilla et al. 2023; Santos et al. 2023). In South Africa, the non-native house sparrows (Passer domesticus) were shown to be significantly heavier, larger and in better condition with increasing urban infrastructure and lower urban vegetation cover, while the Cape sparrow (Passer melanurus) showed opposite trends along urbanization gradient (Naidoo et al. 2024). Urban western fence lizards (Sceloporus occidentalis) in California-USA, have been shown to have shorter limb and toe lengths than their suburban and rural counterparts (Putman et al. 2019). Also, wild boars (Sus scrofa) from an urban area in Spain had larger body size, higher body mass, and better body condition than non-urban wild boars (Castillo-Contreras et al. 2021). Tranquillo et al. (2024a) noted that urbanization had no influence on the body size of Eurasian red squirrels (Sciurus vulgaris), but Eastern grey squirrels (Sciurus carolinensis) were slightly larger in urban than in rural areas of Italy.

Although many studies have been conducted to understand the impact of urbanization on biodiversity at local, regional and global levels to inform the design of appropriate conservation strategies (Robleño et al. 2017; Putman et al. 2019), there are still large knowledge gaps. First, the scientific literature shows geographic bias towards larger metropolitan areas of the Northern hemisphere and Australia (Putman and Tippie 2020; Hahs et al. 2023). Although most biodiversity hotspots are located in the Tropics and Southern Hemisphere where urban growth rate is higher, these areas have been understudied (Tranquillo et al. 2024b). Second, studies on the effects of urbanization on biodiversity are heavily focused on plants and birds (Hahs et al. 2023). Other speciose and functionally important groups such as small mammals and herpetofauna in cities have received less attention. Third, although the functional aspect of biodiversity is crucial, most urban biodiversity studies has focused on taxonomic diversity (Hahs et al. 2023). Besides, the findings of studies that assessed the effects of urbanization on morphological traits of wild animals are largely inconclusive and therefore cannot be generalized. This warrants more studies on diverse taxa and in different geographic locations, particularly in understudied biodiversity hotspots like the Afro-tropics.

Small mammals are diverse, abundant, and inhabit a broad geographic range and habitat types. They serve as consumers, prey and seed dispersers, making them important functional and structural component of natural and semi-natural ecosystems (Boggs et al. 2020). Small mammals have a rapid turnover, are inextricably associated with their environment (Ribeiro et al. 2020; Sobral and de Oliveira 2023), and respond rapidly to environment perturbations through functional and demographic features (Lee et al. 2018; Balčiauskas and Balčiauskien 2022; Slovikosky et al. 2024). Therefore, changes in the structure of small mammal assemblages and morphometric traits over time and space can reveal changes in habitat quality and local disturbance arising from human-induced habitat alterations (Krebs et al 2019; Balčiauskas and Balčiauskien, 2022).

Comparing individuals inhabiting urban areas with conspecifics in rural landscape provides an ideal way to evaluate the effect of urbanization and adaptation mechanisms of small mammals in cities (Tranquillo et al. 2023). Taxonomic diversity (diversity, abundance, species rich and composition) of small mammals have mostly been used to assess the quality of habitats, landscape changes and local disturbances arising from urbanization (Benedek et al. 2021; Mattos et al. 2021; Ofori et al., 2022). However, these measures may not adequately reflect the underlying mechanisms that drive the observed changes at the individual level, or may be insufficient where abundance is higher in low quality habitats. The use of functional traits such as body size, body mass, body condition and other morphometrics in addition to taxonomic diversity, in our opinion, provides a more robust and comprehensive understanding of the mechanisms that drive population dynamics and community structure in cities.

The present study evaluated the effect of urbanization on the diversity, richness, species composition, body size, body mass and body condition of small mammals in the Coastal Savannah Ecological zone (CSEZ) of Ghana. We live-trapped small mammals in two urban sites: farm-bushes (UFB) and a legally protected natural savanna habitat (UPA) within the city of Accra, and compared the data with conspecifics from farm-bushes (RFB) and a legally protected natural savanna habitat (RPA) in rural areas of the CSEZ. Gray’s increasing disturbance hypothesis predicts species loss along the rural-urban gradient (Gray 1989). Therefore, we hypothesized that the urban sites should have lower diversity and species richness compared with the corresponding rural sites. Also, we expected one or few opportunistic habitat generalists to dominant the small mammal assemblages in the urban sites and this should lead to lower species evenness in urban sites compared with their rural counterparts. As diet and habitat specialist species disappear from the urban sites, and opportunistic and habitat generalist species (urban exploiters) move into the urban area, we expected the small mammal species composition to differ between the rural and urban areas. We also expected the urban small mammal assemblage to be nested within the rural small mammal assemblage.

Food availability and predictability can be high in urban areas compared to rural habitats because of human activities that consistently provide reliable food year-round (Meillère et al. 2015). Also, predation pressure has been shown to be low in urban areas due to the reduce number of predators in urban areas compared with rural areas (Gering and Blair 1999; Eötvös et al. 2018). Therefore, we expected the urban small mammal populations to benefit from their environment and hence, have larger body size, body mass and better body condition than their rural conspecifics. Our findings can guide in designing urban landscapes to promote and conserve biodiversity in cities.

Study Area

The study was carried out in urban and rural areas within the Coastal Savanna Ecological zone (CSEZ) of Ghana (Fig. 1), which is one of the dominant ecosystems along the West African coastline. The CSEZ is characterized by a tropical climate with temperatures ranging from about 26 to 31 ˚C and a mean annual temperature of 28˚C. The average annual rainfall is 900 mm, and is bimodally distributed, with mean monthly maxima in May and October (Dowuona et al. 2012). The relative humidity ranges between 59% in the afternoon and 93% at night. The vegetation is typically grassland with substantial amounts of thickets and clumps (Dowuona et al. 2012).

Study sites selection

Small mammals were lived trapped in four sites: (i) urban farm-bushes, (ii) urban legally protected natural savanna vegetation, (iii) rural farm-bushes and (iv) rural legally protected natural savanna vegetation in a rural area, all of which are within the CSEZ of Ghana. The urban farm-bushes were located at the University of Ghana Wacci Teaching and Research Farm and its environs (5.6602° N, 0.1927° W) on the Legon campus in the Greater Accra region. The farm-bushes consist of a mosaic of cultivated land at different stages of crop production and a fallow land that is covered by grass, herbs and shrubs. The crops cultivated on the farm are mainly maize and vegetables (Ofori et al. 2021). The urban legally protected savanna grassland was the Shai Hills resource reserve (5.885° N, 0.05810 E). This protected area covers about 51 km² and is located in the Shai Osudoku District of the Greater Accra region of Ghana (Dowsett-Lemaire and Dowsett 2013). The topography of the reserve consists of a relatively small rocky mountainous area surrounded by a large area of grassland plain. The vegetation is mainly savanna grassland interspersed with shrub, small thickets often around termitaria, and dry gallery forest on the hills and central valley (Ofori et al. 2022).

The rural legally protected area was the Yenku Block B Forest reserve (5.3326 N, 0.6477 W), which forms part of the Muni-Pomadze ramsar site in the Central region of Ghana. The vegetation of the reserve consists mainly of open savanna grassland interspersed with thickets and small trees. The dominant grass and sedge species included Andropogon gayanus, Fimbristylis ferruginea, Heteropogon contortus, Vertiveria fulvibarbis, Panicum maximum, and Sporobolus pyramidalis, while the small trees were mainly Azadirachta indica, Zanthoxylum xanthozyloides, Elaeophorbia drupifera, Cissus argut, and Lonchocarpus cyanescens. The rural farm-bushes (5.5288 N, 0.7061 W) was also located along the central coast and to the west of the Muni-Pomadze Ramsar site. The area consists of a mosaic of active farmland, fallow land and abandoned farm overgrown with grass, herbs, thicket and small trees. The crops cultivated on the active farms include maize, cassava, pepper and okra. Like the Yenku Block B Forest reserve, the dominant grass and sedge species were Andropogon gayanus, Fimbristylis ferruginea, Heteropogon contortus, Vertiveria fulvibarbis, Panicum maximum, and Sporobolus pyramidalis, while the small trees were Zanthoxylum xanthozyloides, Elaeophorbia drupifera, and Lonchocarpus cyanescens.

Small mammal trapping

Small mammals were captured using collapsible Sherman live-traps (23 cm x 9 cm x 7.5 cm, H.B. Sherman Traps Inc., Florida, USA) positioned along three line-transects at each study site. Each transect was about 200 m long, and the distance between consecutive transect at each site was about 100 to 300 m. A total of 20 Sherman traps were placed along each transect, with one trap per trap station, and inter-trap distance of 10 m. The traps were baited with a mixture of peanut butter and corn meal or crushed maize, and were set in the evenings and inspected the following morning between the hours of 7:00 and 9:00 GMT for a total of 30 nights at each study site. There was therefore a total of 1,800 trap-night per site and 7,200 trap-nights for all the four study sites.

Trapped individuals were transferred into a small mesh bag, identified on the spot, sexed using the anal-genital distance (longer in males and shorter in females) and examined for reproductive condition using the presence of either abdominal or scrotal testes in males and enlarged nipples (lactating mother), perforate vaginas (recently mated) and pregnancy in females. Standard morphometric measurements including head and body length (HBL), tail length (TL), right ear length (EL) and right hindfoot length (HFL) were measured for every captured individual using digital vernier calliper to the nearest 0.1 cm. The weight (WT) of individuals was also measured to the nearest 0.1 g using a Pesola Spring balance. Captured animals were assigned to three age categories: adult, subadult and juvenile, based on body size, fur colour and texture. After, their morphometrics were taken, individuals were marked and then release at the point of capture. Small mammal identification followed Happold (2013), Monadjem et al. (2015) and Wilson et al. (2017), while handling protocols followed guidelines of the American Society of Mammalogists (Sikes and Mammalogists, 2016).

Data Analysis

For the diversity measures, we used Shannon-Weiner (H’) and Pielou’s (J’) indices to estimate the species diversity and evenness, respectively. The species richness was defined as the total number of species recorded at each site, and the relative abundance (RA) estimated as the number of individuals of a particular species per the total number of individuals of all species recorded in a site, multiplied by 100%. The similarity of species composition was estimated using the Bray-Curtis quantitative index. To test for the differences in diversity, evenness and species richness among the rural and urban sites, we used the Fisher’s exact test in a 2 × 2 contingency table, with site (rural and urban) as columns and land use type (i.e., farm-bushes and protected area) as rows.

All the morphometrics and body condition comparisons among the rural and urban small mammal populations were performed only on adult animals, excluding pregnant individuals, and for species that occurred in at least a pair of corresponding rural and urban sites (i.e., RFB vs UFB and/or RPA vs UPA), with the number of individuals ≥ 5 per site. We performed combined analysis for males and females, except for Mastomys natelensis, which we analyzed males and females separately because of their greater numbers. Prior to the analysis, the original morphometric data were log₁₀ transformed to normalize the distributions. We estimated body condition using the scaled-mass index (SMI), which has been shown to perform better than most of the body condition indices currently available (Peig and Green 2010). We first tested for differences in mean body size, body mass and body condition among the urban and rural populations using one-way ANOVA. After, we performed a second analysis that adjusted for body size to evaluate the difference in mean body mass among the urban and rural populations using ANCOVA. We set site and sex as fixed factors and body size as the covariate.

We also evaluated the differences in dispersion measures (morphological variation) in a univariate analysis comparing the standard deviation of the body size, body mass and body condition using a variance ratio (F-ratio) test. All statistical analyses were performed using Prima6 software (version 1.0.3) and R (R Core Team 2024), with a significance level of 0.05.

Diversity, relative abundance and species composition

Overall, we captured 505 (without recaptures) small mammal individuals belonging to nine species. A total of 75 individuals comprising five species were captured in the rural farm-bushes (RFB), 105 individuals of six species in the rural protected area (RPA), 288 individuals of seven species in the urban farm-bushes (UFB) and 95 individuals of eight species in the urban protected area (UPA). The species diversity, richness and evenness were slightly higher for the urban sites than the rural sites, but the differences where not significant (Table 1). Mastomys natalensis was the dominant species in RFB, RPA and UFB, forming 69%, 73% and 76%, respectively, at each site. However, in UPA, Tatera kempi was the most abundant species with 44%, while M. natalensis formed only 1% of the total captures. Three species, Taterilus gracilis, Graphiurus nagtglasii and Arvicanthis niloticus occurred in the urban sites only, but all the species recorded in the rural area were also found in the urban area. The dissimilarity of species composition (Fig, 2) between corresponding rural and urban sites was high (RFB vs UFB = 60%, RPA and UPA = 79%). Although the dissimilarity was lowest between the rural sites (RFB vs RPA = 27%), that between the urban sites (UPA vs UFB = 82%) was astonishingly high.

Table 1

Diversity, relative abundance and species composition of small mammals from the rural farm-bushes (RFB), rural protected area (RPA), urban farm-bushes (UFB) and urban protected area (UPA) within the Coastal Savannah ecological zone of Ghana
Species	RFB	UFB	RPA	UPA	p-value
Crocidura olivieri	6 (8)	14 (6)	4 (4)	3 (3)	-
Mastomys natalensis	52 (69)	173 (76)	78 (73)	1 (1)	-
Mus musculoides	10 (13)	33 (14)	3 (3)	5 (5)	-
Lemnyscomys stiatus	2 (3)	0	11 (10)	3 (3)	-
Arvicanthis niloticus	0	36 (16)	0	5 (5)	-
Uranomys ruddi	0	5 (2)	3 (3)	13 (14)	-
Tatera kempi	5 (7)	16 (7)	8 (7)	42 (44)	-
Taterilus gracilis	0	11 (5)	0	0	-
Graphurus nagtglasii	0	0	0	23 (24)	-
Number of individuals (N)	75	288	107	95	-
Number of species (S)	5	7	6	8	1
Pielou's eveness index (J')	0.6225	0.6768	0.5478	0.7466	0.455
Shannon's diversity index (H')	1.002	1.317	0.9815	1.553	0.685
Simpson's index (1-lambda)	0.4966	0.6053	0.4537	0.7272	0.348

Comparison of body size, body mass and body condition between rural and urban sites

Table 2 shows the mean (± S.E) and ANOVA results for the body size, body mass and body condition of Mastomys natalensis from RFB, UFB and RPA, while variation in dispersion of body size, body mass and body condition are shown in Figs. 3, 4 and 5, respectively. We found a significant difference in the mean body size among the three sites for both male and female individuals (Male: F = 11.94, p = 0.00001; Female: F = 0.032). Tukey’s post hoc test showed that males and females from UFB were significantly smaller than their counterparts from RFB (Male: p = 0.02, Female: p = 0.037). Also, males from UFB were significantly smaller than males from RPA (p = 0.00002). In terms of body mass, females (F = 9.858, p = 0.0001) showed a significant difference among the sites. Tukey’s post hoc test revealed that females from UFB were significantly heavier than females from RFB (p = 0.026) and RPA (p = 0.0001). However, after adjusting for body size, the difference in mean body mass was no more significant (F = 0.636, p = 0.426, r² = 0.593, adjusted r² = 0.583). In the case of body condition, we found significant differences among sites for both males and females (Male: F = 5.189, p = 0.0065; Female: F = 18.82, p = 0.0000002). Tukey’s post hoc test showed that males from UFB were in significantly poorer condition than males from RFB (p = 0.024) and RPA (p = 0.009). Similarly, females from UFB were in significantly poorer condition than their counterparts from RFB (p = 0.0014) and RPA (p < 0.00001). The variance ratio test showed no significant difference in dispersion measures (morphological variation) of the body size, body mass and body condition among RFB, UFB and RPA for both male and female M. natalensis individuals (Table 3).

Table 2: Comparison of head and body length (HBL), body mass (BM) and body condition (HFL) of Mastomys natalensis among the rural farm-bushes (RFB), rural protected area (RPA) and urban farm-bushes (UFB) within the Coastal Savannah ecological zone of Ghana

Sex	Variable/Site	Mean ± S.E			ANOVA
Sex	Variable/Site	RFB (n = 31)	UFB (n = 94)	RPA (n = 40)	F	p-value
Male	HBL	11.6 ± 0.3	10.9 ± 0.1	11.9 ± 0.2	11.94	< 0.0001
Male	BM	49.3 ± 3.1	48.4 ± 1.4	44.2 ± 2.8	2.348	0.0988
	BC	48.2 ± 1.6	42.8 ± 1.5	48.2 ± 1.1	5.189	0.007
		(n = 21)	(n = 32)	(n = 38)
Female	HBL	11.8 ± 0.2	11.0 ± 0.2	11.6 ± 0.2	3.546	0.03
Female	BM	46.9 ± 2.7	52.6 ± 2.5	38.8 ± 2.2	9.858	0.0001
	BC	47.4 ± 2.1	38.3 ± 1.5	52.5 ± 2.1	18.82	< 0.0001

Table 3: Comparison of dispersion measures of male and female Mastomys natalensis among the rural farm-bushes (RFB), rural protected area (RPA) and urban farm-bushes (UFB) within the Coastal Savannah ecological zone of Ghana

Parameter	Paired sites	MALE		FEMALE
Parameter	Paired sites	F-ratio	p-value	F-ratio	p-value
Body size (HBL)	RFB vs RPA	1.3	0.26	0.71	0.40
	RFB vs UFB	0.41	0.52	0.59	0.45
	RPA vs UFB	0.53	0.47	0.43	0.51
Body mass (BM)	RFB vs RPA	0.91	0.34	0.76	0.39
	RFB vs UFB	0.61	0.44	1.39	0.24
	RPA vs UFB	0.56	0.46	1.07	0.31
Body condition (BC)	RFB vs RPA	0.89	0.348769	0.78	0.38
	RFB vs UFB	1.17	0.281517	2.17	0.15
	RPA vs UFB	1.05	0.307381	1.7	0.2

For Mus musculoides, we found significant differences in mean body size (HBL: F = 7.429, p = 0.0016), body mass (BM: F = 12.74, p = 0.00004) and body condition (BC: F = 10.83, p = 0.0001) among RFB, UFB and UPA (Table 4). Tukey’s post hoc test indicated that individuals from UFB were significantly smaller than their counterparts from RFB (p = 0.0011). Also, individuals from UFB weighed significantly lesser than those from RFB (p = 0.00006) and UPA (p = 0.0398). The differences in the mean body mass among the sites remained significant after adjusting for body size (F = 3.679, p = 0. 034, r² = 0.511, adjusted r² = 0.439). For body condition, individual from UFB had a significantly poorer condition than those from RFB (p = 0.0005) and UPA (p = 0.016). In terms of variation in dispersion measures (Fig. 3, 4 and 5), the variance ratio test showed no significant difference in morphological variation of body mass and body condition of M. musculoides among the three sites (Table 5). However, for body size, there was a significant difference in the measure of dispersion between individuals from UFB and UPA (F-ratio = 4.51, p = 0.04), but not between individuals from UFB and RFB or UPA and RFB.

Table 4: Comparison of head and body length (HBL), body mass (BM) and body condition (HFL) of Mus musculoides among the rural farm-bushes (RFB), urban rural protected area (RPA) and urban farm-bushes (UFB)

Variable/ Site	Mean ± S.E. (range)			ANOVA
Variable/ Site	RFB (n = 5)	UFB (n = 33)	UPA (n = 5)	F	p-value
HBL	5.9 ± 0.3	5.1 ± 0.1	5.5 ± 0.5	7.429	0.002
BM	7.1 ± 0.8	4.1 ± 0.3	6 ± 0.7	12.74	<0.0001
BC	7.2 ± 0.7	4.3 ± 0.3	6.9 ± 0.7	10.83	0.0001

Table 5: Comparison of dispersion measures of Mus musculoides and Tatera kempi among the rural farm-bushes (RFB), urban farm-bushes (UFB), rural protected area (RPA) and urban protected area (UPA) within the Coastal Savannah ecological zone of Ghana

Parameter	Paired sites	*Tatera kempi*		*Mus musculoides*
Parameter	Paired sites	F-ratio	p-value	F-ratio	p-value
Body size (HBL)	RFB vs UPA	0.47	0.5	0.36	0.56
	RFB vs RPA	14.62	0.003
	RFB vs UFB	0.85	0.37	1.65	0.21
	UPA vs RPA	30.83	<0.0001
	UPA vs UFB	1.79	0.19	4.51	0.04
	RPA vs UFB	0.05	0.83
Body mass (BM)	RFB vs UPA	1.02	0.32	1.28	0.28
	RFB vs RPA	2.25	0.16
	RFB vs UFB	0.38	0.54	1.07	0.31
	UPA vs RPA	2.19	0.15
	UPA vs UFB	0.37	0.55	0.83	0.37
	RPA vs UFB	0.17	0.68
Body condition (BC)	RFB vs UPA	0.09	0.77	0.92	0.35
	RFB vs RPA	0.09	0.77
	RFB vs UFB	0.09	0.77	0.62	0.44
	UPA vs RPA	0.95	0.33
	UPA vs UFB	1.03	0.31	0.67	0.42
	RPA vs UFB	1.08	0.31

Table 6 shows the means and standard errors for body size, body mass and body condition of Tatera kempi from RFB, RPA, UFB and UPA. Our analysis showed significant differences in the mean body size (F = 10.7, p < 0.0001), body mass (F = 3.789, p = 0.014) and body condition (F = 4.29, p = 0.008) among the four sites. Tukey’s post hoc test showed that individuals from UPA were significantly smaller than those from RPA (p = 0.003) and UFB (p = 0.0.0001), but not RFB. For body mass, individuals from RPA were significantly heavier than those from UPA (p = 0.01) and RFB (p = 0.002), but not UFB. However, when body size was accounted for in the analysis, the mean difference in body mass among the sites was not significant (F = 2.093, p = 0.11, r² = 0.314, r² adjust = 0.226). Our analysis also showed that individuals from UPA were in a significantly poorer condition than those from RPA (p = 0.005) and RFB (p = 0.04). Although individuals from UFB (SMI = 90.8 ± 4.6) were in better condition than their counterparts from RFB (SMI = 80 ± 17), the difference was not significant. For variation in dispersion measures (Fig. 3, 4 and 5), the variance ratio test showed no significant difference in the morphological variation of the body mass and body condition of T. kempi among the three sites (Table 5). However, for body size, there was a significant difference in the measure of dispersion between UPA and RPA (F-ratio = 30.83, p < 0.0001) and between RFB and RPA (F-ratio = 14.62, p = 0.003).

Table 6: Comparison of head and body length (HBL), body mass (BM) and body condition (BC) of Tatera kempi among the rural farm-bushes (RFB), urban farm-bushes (UFB), rural protected area (RPA), and urban protected area (UPA)

Variables/ Site	Mean ± S.E. (range)				ANOVA
Variables/ Site	RFB (n = 5)	UFB (n = 16)	RPA (n = 8)	UPA (n = 42)	F	p-value
HBL	13.02 ± 0.6	13.4 ± 0.3	13.3 ± 0.1	11.3 ± 0.3	10.7	< 0.0001
BM	81.4 ± 7.8	92.4 ± 7.0	111 ± 4.1	84.5 ± 2.7	3.789	0.01
BC	80.8 ± 1.7	90.8 ± 4.6	112.0 ± 6.5	86.2 ± 2.8	4.29	0.008

Urbanization often results in declines in species abundance and diversity (Piano et al. 2020) and biotic homogenization (Knop, 2016; Olden et al. 2018; Marcacci et al. 2021; Lokatis and Jeschke 2022; Deng et al., 2024), but there are substantial variations in the spatiotemporal effects of urbanization on wildlife across cities and taxa (Meillère et al. 2015; Putman et al. 2019; Fingland et al. 2021; Hahs et al. 2023). The present study assessed the influence of urbanization on the diversity, species composition, body size, body mass and body condition of small mammals in the Coastal Savannah ecological zone of Ghana. We hypothesized that the urban sites should have lower diversity and species richness compared with the corresponding rural sites, due to the dominance of one or few opportunistic habitat generalist species in the urban sites. For the same reason, we expected the species composition to be different between the urban and rural areas, with the small mammal assemblage in the urban area nested in that of the rural area. We also expected the urban small mammal populations to have larger body size, body mass, and better body condition than their rural conspecifics.

Small mammal diversity, richness and species composition

As expected, our data showed that urbanization altered small mammal species composition, but contrary to our hypothesis, we found that neither the diversity nor species richness of small mammals were significantly higher in rural sites than urban sites. However, when the data was pooled together for the urban (UFB and UPA combined) and rural (RFB and RPA combined) sites, the urban area had three species, Taterilus gracilis, Arvicanthis niloticus and Graphiurus nagtglasii that were not captured in the rural area. Also, when pooled together, all the species recorded in the rural area were found in the urban area, which means that contrary to our hypothesis, the small mammal assemblage in the rural area was perfectly nested within that of the urban area. Our result that urbanization influenced small mammal species composition, but not species diversity and richness, supports the finding of a previous study which also found that urbanization influenced small mammal composition, but not species richness and diversity within forest fragments in the Accra Plains of Ghana (Ofori et al. 2022). Fernández and Simonetti (2013) also found that small mammal composition differed between rural and urban forest fragments within the Chilean Mediterranean zone. However, they observed that species richness was lower in the urban forest fragments.

Our finding that urbanization did not alter the diversity of small mammals in the Coastal Savannah Ecological zone of Ghana supports the results of a meta-analysis assessing mammal abundance and taxonomic diversity in rural and urban sites, which revealed that neither diversity nor abundance was higher in rural than urban habitats (Magura et al. 2021). Contrary to this finding, however, some studies have reported declines in small mammal species richness and diversity along an urbanization gradient (Łopucki et al. 2013; Łopucki et al. 2017). Thus, our findings, together with the results of previous studies, seem to suggest that there are no consistent species richness and diversity patterns in terrestrial mammal assemblages along rural-urban gradients.

From a landscape perspective, the effect of urbanization, whether positive or negative, depends on the habitat amount hypothesis (Fahrig 2013; Martin 2018; Watling et al. 2020) and the structure, composition and configuration of the landscape (Huang et al. 2023). The environmental factors that explain biodiversity patterns in cities are mainly temperature, precipitation, primary productivity (energy availability). For small mammals, the proportion of suitable habitats in quantity and quality, and habitat heterogeneity are particularly important in explaining their taxonomic diversity (Melo et al. 2017; Rowland-Schaefer et al. 2024). The city of Accra has many urban green areas with varying sizes, shapes, and habitat quality, which might have accounted for the comparable species diversity and richness between the urban and rural sites. Also, the novel characteristic of the urban environment, including reliable year-round food availability and relatively high ambient temperatures resulting from the urban heat effects, may be serving as a filter selecting for some small mammals from the regional species pool that are selected against in the rural area. This could explain the variation in species composition between the urban and rural habitats.

Body size, body mass and body condition

Body size, body mass, and body condition are ‘ecomorphs’ that have fitness implications for small mammals because they correlate with aspects of their ecology and life history traits, such as reproduction rate and development time (Putman et al. 2019). Our data revealed that urbanization had effects on the body size, body mass and body condition of small mammal. Mastomys natlensis, M. musculoides and T. kempi individuals from the urban sites had smaller body size and were in poorer condition compared with conspecifics from the corresponding rural sites (urban farm-bushes vs rural farm-bushes, and urban protected area vs rural protected area). The effect of urbanization on body mass was observed for M. musculoides only, with individuals from UFB weighed significantly lesser than those from RFB. The variance ratio test showed significant difference in dispersion measure of body size between M. musculoides individuals from UFB and UPA, but not between an urban and its corresponding rural site. However, for T. kempi, there was a significant difference in the measure of dispersion between UPA and RPA and between RFB and RPA.

Previous studies on the effects of urbanization on small mammal morphometrics showed contrasting patterns between species or even among individuals of the same species studied in different geographical areas (Tranquillo et al. 2024b). For example, urbanization did not influence the body size of the Eurasian red squirrel (Sciurus vulgaris), but Eastern grey squirrels (Sciurus carolinensis) were slightly larger in urban than in rural areas in Italy. However, both the red greys squirrels were heavier in the urban than in rural sites (Tranquillo et al. 2024a). This result is completely opposite from our finding that urbanization influenced the body size of small mammals, with individuals in the urban area having significantly smaller size, and that urbanization did not affect the body mass of M. natalensis and T. kempi. Although urbanization influenced the body mass of M. musculoides, individuals in the urban site were significantly lighter than those in rural site.

Using a densely sampled, multi-source dataset to assess the effects of climate and urbanization on body size of Peromyscus maniculatus across North America, Guralnick et al. (2020) found that the body size of the species was smaller in more urbanized areas, just as we found for M. natlensis, M. musculoides and T. kempi in the present study. In accordance with our finding on small mammal body mass for M. natalensis and T. kempi, Shimamoto et al. (2020) did not find any significant difference in body mass of Eurasian red squirrels between urban and rural sites. Schmidt et al. (2022) also did not find any significant difference in body mass of Eastern grey squirrels (Sciurus carolinensis) between an urban area and a rural forest. However, contrary to our finding that small mammals in the rural area were in significantly better condition, Schmidt et al. (2022) did not find any significant difference in the body condition of Eastern grey squirrels between the urban area and rural forest. A comparative study on chipmunks (Tamias striatus) reported heavier females in urban than in natural populations, but no differences in body mass among males (Lyons et al. 2017). However, in the present study both male and female M. natalensis individuals did not show any significant difference in body mass between the urban and rural areas.

Investigations of urbanization effects on the morphometrics of other terrestrial vertebrates have also yielded varying trends and directions. For instance, Naidoo et al. (2024) found that the non-native house sparrow (Passer domesticus) was significantly heavier, larger and in better condition with increasing urbanization and lower urban vegetation cover, while its native congener, the Cape sparrow (Passer melanurus), showed opposing trends along these gradients in South Africa. Urban western fence lizards (Sceloporus occidentalis) in California-USA, have been shown to have shorter limb and toe lengths than their suburban and rural counterparts (Putman et al. 2019). A qualitative synthesis of the literature, and a quantitative phylogenetic meta-analysis comparing phenotypic traits between urban and non-urban lizard populations revealed that urban lizards are larger than their non-urban counterparts (Putman and Tippie 2020). Castillo-Contreras et al. (2021) noted that wild boars (Sus scrofa) from an urban area in Spain had larger body size, higher body mass, and better body condition than non-urban wild boars.

The smaller body size and poorer body condition of small mammals in the present study could be explained by the Bergmann’s rule (Bergmann 1847) and credit-card hypothesis (Shochat 2004). The Bergmann’s rule postulates that thermoregulatory response associated with warmer climate drives decreases in body size (Atkinson 1994). The city of Accra is consistently warmer than its surrounding rural habitats because of urban heat island effect. Small mammals might be responding to the urban heat effect by reducing their body size and fat reserves in order to compensate for the energetic costs of metabolism and thermoregulation associated with being ‘large and chubby’ in a warm environment. The credit-card hypothesis proposes a decrease in body size, body mass, and body condition with increasing urbanization because of increasing food availability (Shochat 2004). The availability and predictability of food resources in urban areas are higher compared to rural or natural habitats because human activities provide reliable food resources year-round in urban areas. Intuitively, one would expect that small mammals in the urban area will benefit from the high food availability to enhance their body size, body mas and body condition. However, counter-intuitively, the constant year-round access to food in urban areas reduces the necessity for individuals to accumulate fat reserves, leading to small body size, body mass and poor body condition compared to rural conspecifics that have to accumulate fat reserves during times of high food availability (typically during the wet season) to buffer against unpredictable temporary food shortages in the dry season. Also, the accumulation of body fat reserves is associated with high metabolic and locomotory costs. Therefore, in the urban area where ambient temperature is relatively higher compared to rural areas, being small and lean could be advantageous. It may also be that the available food resources in the urban area are of poor nutritional quality and therefore the small body size, body mass and body condition of small mammals in the urban area could be due to energetic constraints in the city.

Our study demonstrates the effects of urbanization on taxonomic diversity and morphometrics of small mammals in the Coastal savannah Ecological zone of Ghana. Body size, body mass and body condition in vertebrates correlate directly with fitness (survival, growth and reproductive success). Intuitively, the smaller body size, body mass and poorer body condition of the small mammals in the urban area may seem to compromise their fitness in the city. However, this could be an adaptive response to the predictable year-round food availability in the urban area, and also to reduce metabolic stress imposed by the urban heat effect. Our study is the first to simultaneously assess the effects of urbanization on the taxonomic diversity and morphometrics of non-volant small mammals in Ghana and perhaps West Africa in recent times. Our data adds to the relatively scant global literature on the response of small mammals to urbanization in the Afro-tropics. Our findings revealed that green areas within the urban landscape can maintain small mammal species richness and diversity that are comparable to natural or rural habitats. It is therefore important for urban developers and land use planners to prioritize the integration of green areas into urban planning designs in order to promote and conserve wildlife in cities.

Ethical Approval

The trapping and handling of small mammals were conducted following humane procedures and established protocols of the College of Basic and Applied Sciences Ethics Committee of the University of Ghana, Legon (Ref. no. ECBAS 024/19). Additional permission for the research in protected areas was granted by the Wildlife Division of the Forestry Commission of Ghana, and verbal consent was sought from the farmers whose farms were included in the study.

Conflicts of Interest

The authors declare that there are no conflicts of interest regarding the publication of this paper.

Authors’ Contributions

All authors contributed equally to data collection, analysis and writing and editing of the manuscript.

Data availability

Data will be made available upon request.

Acknowledgments

The authors gratefully acknowledge the support of Jones K. Quartey, Helen S. Addom, John B. Mensah and all the park officials at Shai Hills Resource reserve and Muni-Pomadze Ramsar site who supported us with our data collections.

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Effects of urbanization on taxonomic diversity, body size, body mass and body condition of non-volant small mammals in the Coastal Savannah Ecological zone of Ghana

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Abstract

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INTRODUCTION

MATERIALS AND METHODS

Study Area

Study sites selection

Small mammal trapping

Data Analysis

RESULTS

Diversity, relative abundance and species composition

Comparison of body size, body mass and body condition between rural and urban sites

Discussion

Small mammal diversity, richness and species composition

Body size, body mass and body condition

Conclusion

Declarations

References

Additional Declarations

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